1. Field of the Invention
The present invention relates to a method of synthesizing single-crystal KTiOPO.sub.4 by a flux process.
KTiOPO.sub.4 (KTP) conventionally has been prepared by a hydrothermal method of synthesis. This method requires a high pressure of at least 1,000 atm to achieve growth, and thus requires special and expensive apparatus. Moreover, the use of water presents the added problem that optical transparency of KTiOPO.sub.4 is reduced because of absorption by the OH groups contributed by the water.
As an alternative to the hydrothermal method, growth by a convenient flux method was very recently studied. While several approaches have been proposed, Bell Telephone Laboratories announced a promising technique in 1986. According to their method, potassium phosphate is mixed with titanium oxide and tungsten oxide (the latter as a flux to promote fusion of the remaining materials), and the mixture is heated to about 1,000.degree. C. 3K.sub.2 WO.sub.4.P.sub.2 O.sub.5 forms in the flux by the following reaction: EQU 4K.sub.2 HPO.sub.4 +2TiO.sub.2 +3WO.sub.3 .fwdarw.2KTiOPO.sub.4 +3K.sub.2 WO.sub.4.P.sub.2 O.sub.5 +2H.sub.2 O.uparw. (1)
When the resulting solution is slowly cooled, KTiOPO.sub.4 will form by itself.
Another version of the flux method is described in the specification of unexamined published Japanese Patent Application No. 62-27314. According to this method, a crucible placed in a furnace is charged either with a mixture of titanium oxide and oxides of components of the intended compound, or with precursors of these oxides or salts thereof, or with a previously synthesized compound of the KTP type. The charged mixture or compound is heated to a given temperature in the range of from about 650.degree. C. to about 1,100.degree. C. so as to dissolve the mixture or compound in at least one alkali metal halide to be introduced into the crystal. Subsequently, the crucible is cooled almost to room temperature at a rate no faster than about 50.degree. C. per hour. The formed glass crystal is then separated from the reaction mixture.
The methods described above present various problems. These problems relate to contamination by the flux or to low purity of starting materials.